Princess Bride.The book is virtually unreadable crap. The movie is magical. Perfect cast, great story, sly humor.
Stardust. Gaiman was involved in the movie adaptation, and it has significant changes from the book that tidy up the story and fit the film genre better. The book is a bit darker, but by no means his darkest stuff.
The Maltese Falcon (1941). Hays code forced the film to lighten a lot of the rough edges of the book, but the cast was perfect an the cinematography outstanding. The perfect noir mystery.
Something Wicked This Way Comes. A slight cheat, as Bradbury wrote the script in 1958, and failing to get it filmed turned the story into a novel in 1962 before Disney optioned the original screenplay (which was rewritten at one point behind Bradbury's back). But despite the watering down of the darker themes and the tension during production, it is a decent film of one of my favorite books.